In a hearing aid, acoustical signals arriving at a microphone of the hearing aid are amplified and output with a small loudspeaker to restore audibility. The small distance between the microphone and the loudspeaker may cause feedback. Feedback is generated when a part of the amplified acoustic output signal propagates back to the microphone for repeated amplification. When the feedback signal exceeds the level of the original signal at the microphone, the feedback loop becomes unstable, possibly leading to audible distortions or howling. To stop the feedback, the gain has to be turned down.
The risk of feedback limits the maximum gain that can be used with a hearing aid.
It is well-known to use feedback suppression in a hearing aid. With feedback suppression, the feedback signal arriving at the microphone is suppressed by subtraction of a feedback model signal from the microphone signal. The feedback model signal is provided by a digital feedback suppression circuit configured to model the feedback path of propagation along which an output signal of the hearing aid propagates back to an input of the hearing aid for repeated amplification. The transfer function of the receiver (in the art of hearing aids, the loudspeaker of the hearing aid is usually denoted the receiver), and the transfer function of the microphone are included in the model of the feedback path of propagation. Thus, the feedback suppression circuit adapts its transfer function to match the corresponding transfer function of the feedback path as closely as possible.
The digital feedback suppression circuit may include one or more digital adaptive filters to model the feedback path. An output of the feedback suppression circuit is subtracted from the audio signal of the microphone to remove the feedback signal part of the audio signal.
In a hearing aid with more than one microphone, e.g. having a directional microphone system, the hearing aid may comprise separate digital feedback suppression circuits for individual microphones and groups of microphones.
Ideally, the feedback part of the audio signal is removed completely so that only an external signal generated in the surroundings of the hearing aid is amplified in the hearing aid. In practice, however, the feedback suppression circuit cannot model the feedback path perfectly; leaving an undesired residual feedback signal for amplification. Near instability, the residual feedback signal may cause the hearing aid output level to exceed the desired output level.
EP 2 203 000 A1 discloses a hearing aid with suppression of residual feedback utilizing an adaptive feedback gain circuit wherein the level of residual feedback is estimated based on the hearing aid gain and a feedback path model as determined during power up or during fitting of the hearing aid.